Zinc-oxide structure joins nanoscale-device push

HANCOCK, N.H.  Nanorings, a new type of geometry in the quest to build nanoscale devices, hold out near-term promise as injectable pressure sensors to monitor the human body.

The uniform, crystalline zinc-oxide rings, 1 to 4 microns in diameter and 30 nanometers thick, were produced by a research team at the Georgia Institute of Technology's Center for Nanoscience and Nanotechnology (Atlanta).

While the geometry simply joins nanowires and nanotubes in a line of building blocks for nanoscale devices, lead researcher Zhong Lin Wang called the piezoelectric nature of zinc-oxide "a dramatic advance." Wang believes a new class of nanoscale devices with applications in MEMS and biotechnology will result from the discovery.

Put under stress, piezoelectric materials produce an electric current or, conversely, re-spond to an electric field by changing shape. That allows the rings to function as small-scale pressure and force sensors, opening a novel potential application as inject-able pressure sensors for monitoring the body.

One near-term application, for example, would be injectable pressure sensors that could monitor blood pressure at specific sites in the body in real-time. Fortunately, zinc-oxide is compatible with the body and is a required nutrient, Wang said. The rings also could function as tiny fluid pumps for lab-on-a-chip systems.

Wang's group accidentally discovered how rings form as it tried to build nanoscale piezoelectric devices. "We were puzzled to find that zinc-oxide belts would roll up into complete and closed rings during the growth process," he said. The cause was simple. Opposite charges build up on opposite sides of the belt structure, and electrostatic attraction causes the flat layer of zinc-oxide crystal to curl around in a spiral pattern to neutralize the force. The edges then grow together to form a perfect crystalline ring.

Seamless single-crystal nano-ring has a diameter of 3 microns.

The same phenomenon should occur in materials with a structure similar to zinc-oxide. Known as wurtzite-structured material, they are characterized by an asymmetrical structure and surfaces that become electrically polarized. "The intrinsic polar surface not only induces unique asymmetric growth behavior and new hierarchical self-assembling structures, but also provides a new facet in understanding fundamental growth phenomena. The results received from ZnO should impact the growth of nanostructures for the entire wurtzite family," Wang said. Other materials in that class are gallium-nitride, aluminum-nitride, indium-nitride and zinc-sulphide.

Wang plans to develop the nanoring process by integrating the devices into both silicon-based microsystems and biotechnology systems. The nanoscale piezoelectric belts could also be useful in MEMS, he said.